Its a little more complicated than that, the simple answer is at least 3. But because today’s modern telephones have lower requirements for ringers than the ‘standard’ bells of days gone by, you can usually do more.
In order to understand just how many, we need to understand RENs (which are usually on a label on the phone). The following is a good guide to figuring out just what you can use.
Ringer equivalence number (REN)
While REN is probably not something that will come into your decision process I want to let you know how it affects your telephone performance. REN is a measurement of how much ringing power certain telephone equipment takes or generates (in this case the ATA generates the ring). From the ATA side REN determines how many telephones can connect to each ATA line and still get them ringing properly.
SPA-2100 Max Ringer load = 3REN
PAP2 Max Ringer load=5REN
In a case where you plan to connect the ATA to a line port on your key system or to a single phone the difference in REN values are meaningless. However, if you trying to drive 3 phones bridged to a single ATA port, REN value can be a factor. You can usually find the REN use value of your telephone on a label on the bottom of the phone.
Just add up all the RENs on the phones, and be sure that the value is less than (or equal to ) of the adaptor you have. So if you have a total REN for all your Phones of 3.5, you would be able to use a PAP2, but not an SPA2100.